Motor propelled heavy machinery, such as track-driven earth movers and other construction machinery, are sometimes required to operate on relatively steep slopes. When operators cause such vehicles to descend a steep grade, a variety of braking and arresting mechanisms are potentially activated to prevent out-of-specification operation leading to machine damage. Examples of potentially damaged components include braking systems as well as operating speed-sensitive components such as an engine, pump and/or hydraulic motor.
A number of control schemes have been proposed for controlling downhill machine speed on severe grades. For example, in Sychra et al., U.S. Pat. No. 7,460,941 a downhill incline and machine ground speed are determined and an amount of braking required to prevent the machine from exceeding a speed limit for the measured speed. In another example of a control scheme for a machine, a target speed is determined. Furthermore, a trigger condition is based upon a current grade exceeding a threshold. In response, controls are activated to prevent the machine from exceeding the target speed. Such controls include an engine brake and a transmission retarder.
One way to ensure that a machine does not encounter overspeed operating conditions that may harm moving parts of the engine and power train is to tune the machine to operate, once activated, according to a worst-case downhill slope scenario (e.g., traveling down the steepest anticipated slope, for example). However, operating a machine configured according to this assumed worst case slope leads to excessive machine braking on less steep slopes. This, in turn, leads to limit cycling of the control (where a repeating on/off behavior emerges as the machine is slowed excessively after the triggering condition is reached, then allowed to accelerate when the triggering condition no longer holds).
This and other shortcomings in the state of the art are addressed by aspects of an exemplary method and variable transmission assembly (including a controller thereof) described herein.